What is the role of a computed tomography (CT) scan of the lower extremity in evaluating suspected cellulitis, particularly in patients with signs of severe infection or impaired renal function?

CT Imaging for Lower Extremity Cellulitis

CT scanning is NOT routinely indicated for evaluating uncomplicated cellulitis and should be reserved for specific clinical scenarios where deeper infection, abscess, or necrotizing fasciitis is suspected. 1

When CT is Appropriate

CT with IV contrast has a role when clinical diagnosis is uncertain and deeper pathology must be excluded:

• Suspected necrotizing soft tissue infection (NSTI) - CT demonstrates 100% sensitivity for identifying NSTI, with findings including fat stranding, fluid and gas collections dissecting along fascial planes, fascial thickening, and non-enhancing fascia suggesting necrosis 1

• Deep soft-tissue abscesses or sinus tracts - CT is helpful for detecting collections not apparent on physical examination, particularly in diabetic foot infections where deeper involvement may be present 1

• Failure to respond to appropriate antibiotic therapy - When cellulitis does not improve after 48 hours of beta-lactam treatment, imaging may reveal occult abscess, osteomyelitis, or necrotizing infection 1

• Signs suggesting deeper infection - "Wooden-hard" subcutaneous tissue, pain disproportionate to examination findings, violaceous bullae, cutaneous hemorrhage, skin sloughing, skin anesthesia, rapid progression, or suspected gas in tissue warrant urgent CT evaluation 1, 2

Critical Limitations and Caveats

Imaging studies must NEVER delay surgical consultation when necrotizing infection is suspected - clinical judgment remains paramount, and requesting CT may delay life-saving debridement 1

• Plain radiographs have poor sensitivity and should not be used to rule out necrotizing infection, though subcutaneous gas when present is highly specific 1

• Bedside ultrasound is preferred over CT in unstable patients - ultrasound can differentiate simple cellulitis from necrotizing fasciitis with 88.2% sensitivity and 93.3% specificity, and can be performed without transporting critically ill patients 1

• MRI provides superior soft tissue detail but is impractical in emergency settings and not recommended as first-line imaging 1

Standard Cellulitis Does Not Require Imaging

The diagnosis of typical cellulitis is clinical - based on acute onset of erythema, warmth, swelling, and tenderness 1, 3, 4

• Blood cultures and imaging are unnecessary for patients with uncomplicated cellulitis who lack systemic toxicity 1

• Laboratory parameters (CBC, CRP, creatinine, CPK) should be obtained only when systemic signs are present: fever/hypothermia, tachycardia >100 bpm, hypotension <90 mmHg, or altered mental status 1

Renal Function Considerations

In patients with impaired renal function (elevated creatinine), hospitalization should be considered and imaging pursued more aggressively - elevated creatinine is a marker of severe infection requiring definitive diagnosis through CT, surgical exploration, or tissue biopsy 1

• The combination of hypotension AND elevated creatinine mandates aggressive evaluation including imaging and surgical consultation 1

• CT with contrast can be performed with appropriate precautions in renal impairment, though contrast-induced nephropathy risk must be weighed against diagnostic necessity 1

Practical Algorithm

1. Assess for systemic toxicity - fever, hypotension, tachycardia, altered mental status 1, 2
2. Examine for warning signs of deeper infection - severe pain, wooden-hard tissue, bullae, rapid progression 1, 2
3. If unstable or necrotizing infection suspected - perform bedside ultrasound immediately and obtain emergent surgical consultation WITHOUT waiting for CT 1
4. If stable but diagnosis uncertain - CT with IV contrast can differentiate cellulitis from abscess or NSTI 1
5. If typical cellulitis presentation - treat empirically with beta-lactam antibiotics without imaging 1, 2, 3